The present invention relates to an optical printer for e.g. a laser beam printer in which a rotating mirror is used to scan a light spot in a main scanning direction, and more particularly to an optical scanning device suited to a printer for a facsimile.
The performance of the optical scanning device used in e.g. a laser beam printer is greatly affected by the accuracy of correcting the distortion aberration involved with the linear scanning on a scanning face and the mechanical accuracy of a rotating mirror.
FIG. 25 shows a general arrangement of the optical scanning device. The light emitting from a laser light source 102 consisting of a laser, a coupling lens, a slit, etc. is focused by a cylindrical lens 103 in its sub-scanning direction, deflected by a scanning mirror which is driven by a scanning (rotary driving) motor 104 and scanned on the surface of a photosensitive member 109 (main scanning).
The cylindrical lens 103 and first and second f-.theta. lenses 106 and 107 are provided to make optical correction so that the above optical scanning is correctly made on the photosensitive member 109. More specifically, the cylindrical lens 103 and first and second f-.theta. lenses 106 and 107 are used in order to correct the scanning line shift necessarily generated although the scanning motor 104 and scanning mirror 105 are fabricated with high accuracy while the first and second f-.theta. lenses 106 and 107 are used to correct the image field curvature distortion and distortion aberration generated owing to linear scanning.
On the other hand, a synchronization sensor 108 is provided to detect the scanning position of the photosensitive member 109 so that the light is correctly scanned.
FIG. 26 shows the laser light-emitting signal emitting from the laser light source 102. The laser light source 102 emits an image signal 1102 at the timing when a synchronous signal 1101 has been received by the synchronization sensor 108. Incidentally, the image signal 1102 is composed of image ON signals 1102 and image OFF signals 1103 which have fixed periods, respectively.
As described above, generally, the distortion aberration in the optical scanning device is corrected using the f-.theta. lenses as shown in FIG. 25, but in addition to this technique, the method of making the light emitting timing variable is disclosed in JP-A-55-118012. FIG. 27 schematically shows the manner of light emission. If correction of the distortion aberration by means of the f-.theta. lens is not made, with the incidence angle of .theta.=0 at the center of the photosensitive member 109, the light scanned by the scanning mirror of FIG. 25 will become coarse with an increase of the distance y=tan .theta. from the center to the end. In order to obviate such a difficulty as shown in FIG. 27, the light emission is corrected so that it is dense at the center and coarse at the end.
The above rotating angular velocity control and the light emission timing/light amount control may be combined. This can be performed by the following equation representative of a motor control movement: EQU I(d.omega./dt+C.omega.=T.sub.0 +T.sub.1 e.sup.jn.omega.t
I: inertial moment of the motor and mirror PA1 C: bearing/windage loss PA1 T.sub.0 : normal torque constant PA1 T.sub.1 : control torque constant PA1 n: number of mirror faces
Now if the angular velocity is varied by .+-.10%, the torque to be changed is 2-10 times as large as the normal torque, thus leading to a problem of endurance. In order to relax such a load, the control of timings of light emission is also used. Specifically, as shown in FIG. 30, in order to make uniform the spot size at the light receiving unit, the timing of light emission is made dense at the end and coarse at the center.
The other means of controlling the light emission is to change the rate of ON to OFF, i.e. duty, in one period of the strength of light power in the emitted light waveform shown in FIG. 30.
In order to fix the amount of received light, control is made so that the product of the time of light emission and the optical power is constant. However, if a saturated response area where sensitivity is saturated for light amount in the characteristic curve of the light amount vs. the sensitivity as shown in FIG. 31, is used, the optical power can be made constant without changing the light amount. Therefore, this prior art has a merit of simplifying a lens system.
Meanwhile, the scanning motor 104 is required to rotate at a fixed speed with high accuracy. In order to satisfy this requirement, the light receiving timing of the synchronization sensor 108 is fed back to a fixed speed control unit 1301 so that the electric power to be supplied to the scanning motor 104 is adjusted to provide the fixed speed.
FIG. 29 is a general arrangement of the fixed speed control unit 1301 of FIG. 28. The signal inputted from the synchronization sensor 108 and the reference signal 1501 are compared by a frequency/phase comparator unit 1502, and necessary electric power is supplied from an electric power adjustment/output unit 1503 to the scanning motor 104, thus realizing the fixed speed of the motor.